In the electrolytic process of graining a lithographic plate, it has been observed that a layer of heavy, dark smut is usually deposited upon the surface of the plate after subjecting the plate to an alternating current at a current density below 15 amps per square decimeter (A/dm.sup.2) and at a concentration greater than 10 grams per liter of acid, especially hydrochloric acid. It is generally believed that smutting is caused by the increased surface area produced by the graining process. It would be desirable to eliminate or reduce the smut build-up. Reducing or eliminating the smut would make the electrolytic process more cost-effective by eliminating the additional steps presently required to clean the grained surface.
In order to achieve a desirable grain topography in aluminum plate an improved electrochemical surface process has been developed wherein the alternating current wave form is varied to provide a greater anodic phase. Also, such wave shaping has provided a pause in the current between anodic and cathodic phases. The above process is described in the U.S. Pat. No. 4,294,672, issued Oct. 13, 1981 to Ohba et al.
The improved graining process utilizes an alternating current of standard frequency of 50 to 60 Hz. Some mention is made that higher frequencies have been observed to provide pits of smaller size in the aluminum surface. However, the use of higher frequencies has not been suggested as a means of eliminating or reducing smut, and there is no disclosure of at what current density pit formation occurs. In fact the increase in the surface area achieved by the production of the small grain size would lead those skilled in this art to expect a heavier and darker smut layer to be formed. In the improved process as disclosed in the prior art, the electrolytically grained plates are routinely desmutted with nitric acid, sodium bisulfate, or alkaline solution.
In U.S. Pat. No. 4,297,184 issued Oct. 27, 1981 to Dyer, mention is made of a process using lowered alternating current frequencies of 15 and 25 Hz. Wave shaping is also employed to provide etched aluminum foil of greater capacitance. There is, however, nothing whatsoever in this patent about the effect of the lowered frequency upon the formation of smut. U.S. Pat. No. 4,376,686 issued Mar. 15, 1983 to Arora, mentions low AC frequency of 12 to 20 Hz for aluminum foil of higher capacitance, nothing is said about smut formation.